Invalidity from nonparallelism in a radioimmunoassay for erythropoietin accounted for by human serum antibodies to rabbit IgG.

An immunologic crossreactant of erythropoietin seemed to develop and persist in serum samples from a patient during treatment and remission of idiopathic aplastic anemia. It had a steeper slope to radioimmunoassay log-dose response lines and a larger molecular size than erythropoietin. On fractionation of serum, the apparent crossreactant was bound by staphylococcal Protein A at pH 7.5 and recovered by elution from it at pH 3.0. Adsorption of serum from the patient, and from one of two similarly affected children, with rabbit IgG linked to agarose appeared to remove completely the apparent crossreactant. These treated sera gave radioimmunoassay log-dose response lines essentially parallel to that given by the International Reference Preparation (IRP) for erythropoietin and estimates of immunoreactive erythropoietin appropriate to the normal hemoglobin concentrations. The apparent crossreactant of erythropoietin is thus accounted for by heterophilic antibodies to rabbit IgG. These developed in the patient following treatment with rabbit antilymphocyte globulin but seem to have arisen spontaneously in the children. Thus iatrogenic and idiopathic antibodies to rabbit IgG interfered in a radioimmunoassay for erythropoietin in serum through their ability to react with the radioimmunoassay anti-erythropoietin antiserum raised in rabbits.